1. Field of the Invention
The present invention relates to a linear motor used in an X-Y stage of, for example, a semiconductor exposure apparatus or a form determining apparatus, or used in a precision positioning device of, for example, a high-precision processing machine. The present invention also relates to a stage device and an exposure apparatus using the linear motor.
2. Description of the Related Art
In semiconductor exposure apparatuses, form determining devices, and high-precision processing machines, there is a demand that anything to be processed, such as a wafer to be exposed, or anything to be measured be quickly and very precisely positioned. To respond to such a demand, progress is being made in the development of an X-Y stage or the like provided with a linear motor serving as a driving section which can allow precise positioning and has excellent responsiveness.
FIG. 22 is a view showing a generally-used reduction projection exposure apparatus comprising components such as a wafer stage (XY stage) E used for positioning a wafer W, a projection optical system A disposed thereabove, a reticle stage B, and a light source optical system C. Exposure light from the light source optical system C passes through a reticle on the reticle stage B in order to form an image thereof on the wafer W by means of the projection optical system A, whereby the reticle pattern is transferred onto the wafer W.
The wafer stage E is disposed above a table 110 in order to support the projection optical system A and the reticle stage B, with a body frame D rising from the table 110. Vibration removal devices H are provided between a base G, which supports the table 110, and a floor surface F.
The position of the wafer stage E is measured using a laser interferometer J, and the measured position is fed back to a control system of the wafer stage E. The light source optical system C is supported by a light source support K which rises directly from the floor surface F.
The wafer stage E is an X-Y stage comprising components such as a Y stage capable of reciprocating freely above the table 110 in a Y-axis direction, an X stage capable of reciprocating freely with respect to the Y stage in an X-axis direction, a Y drive section for moving the Y stage in the Y-axis direction, and an X drive section for moving the X stage in the X-axis direction.
The Y drive section and the X drive section for moving the Y stage and the X stage, respectively, are each provided with a linear motor of the type shown in FIGS. 20 and 21. The linear motor 140 comprises a movable element 141 integrally connected to the Y stage and the X stage, and a stator 142 extending through an opening in the movable element 141. The stator 142 comprises a coil row 142a and supports 142b for supporting the coil row 142a. The movable element 141 has a hollow frame comprising a pair of opposing iron plates (i.e, a yoke) 141b for holding a magnet 141a and a pair of aluminum plates 141c affixed to both ends of the iron plates 141b.
When current is supplied to each of the coils of the coil row 142a of the stator 142 of the linear motor 140, a thrust is produced due to the Lorentz force, causing the movable element 141 to move along the coil row 142a.
In the prior art, however, when, as mentioned above, current is supplied to the row of coils 142a, causing the linear motor 140 to be driven, the coils get heated. The heated coils heat structures therearound and the atmosphere, thereby reducing the positioning accuracy of the X-Y stage.
More specifically, part of the heat produced in the coils of the linear motor is transmitted to the support of the row of coils and reaches such structures as the body frame of the exposure apparatus, causing thermal deformation of these structures. The rest of the heat of the coils heats the atmosphere, causing the laser beam path of the laser interferometer to shake, as a result of which, errors occur in the measured values of the laser interferometer.
It has been experimentally found that a change of only 1.degree. C. in the temperature of the atmosphere of the laser beam path of the laser interferometer produces an error of 100 nm in the measured value.
Specifically, in an X-Y stage of a semiconductor exposure apparatus which is required to have a positioning accuracy on the order of nanometers (nm), if the structures around the linear motor are, for example, 100 mm in length and have a coefficient of thermal expansion on the order of 1.times.10.sup.-6, a 1.degree. C. change in temperature causes a thermal deformation of 100 nm, so that such heating of the linear motor is a great obstacle in obtaining the required positioning accuracy.
The heating of the linear motor may also heat, for example, the wafer on the X-Y stage or the object to be measured, thus reducing the processing accuracy or the measuring accuracy.
An attempt has been made to overcome the problem of reduced positioning accuracy due to heating of the coils by the use of a cooling jacket or the like that forcefully cools the coils of the linear motor, as disclosed, for example, in Japanese Patent Laid-Open Nos. 7-302124, 7-302747, and 8-167554. However, when the cooling jacket is made of a non-insulating material, eddy currents are produced with the movement of the magnet and the yoke of the movable element of the linear motor, resulting in an increase in the so-called viscous resistance which opposes the driving force of the linear motor. Thus, the driving efficiency of the linear motor is reduced.
Further, when the flow rate of the cooling medium is increased to increase the cooling capability, the pressure of the cooling medium is increased. This increase in pressure outwardly deforms the thin portion of the jacket, which may cause it to contact the permanent magnet or cause it to break. To prevent such problems, it is necessary to sufficiently strengthen the thin portions of the jacket by the proper amount. On the other hand, it is necessary to decrease the distance between the permanent magnets in order to increase the magnetic flux density. Thus, there is a demand to make the thin portions thinner, to the extent possible, to make the jacket smaller.